Stung by attacks that he was being too soft, Sir John Chilcot hit back. "We have not been trying to ambush witnesses or score points," he said. "We are not here to provide public sport or entertainment. The whole point of our approach has been to get to the facts."

That was before Christmas. His critics have since been largely silenced as the inquiry has been given illuminating, at times provocative, insights.

This week, it heard Alastair Campbell, Tony Blair's spin doctor, insist he stood by "every single word" of the now discredited Iraq WMD dossier, which even officials responsible for drawing it up, including Sir John Scarlett, chairman of the joint intelligence committee, did not defend.

The next day Lord Turnbull, the cabinet secretary at the time, provided a refreshing antidote. He described Campbell's attacks on Clare Short, the then international development secretary, for being untrustworthy as "very poor".

In evidence as robust as anything Campbell offered, Turnbull praised Robin Cook, who resigned in protest at the imminent invasion. Cook was "absolutely spot on", said Turnbull. Cook died in 2005.

The inquiry is about to face its big test, whether its calm inquisitorial process is better than a more adversarial one, as it prepares next week to question Jonathan Powell, Blair's chief of staff, and Geoff Hoon and Jack Straw, then defence and foreign secretaries. They will be followed by Lord Goldsmith, then attorney general, and Elizabeth Wilmshurst, deputy chief legal adviser at the Foreign Office, who also resigned in protest at the invasion. Then it will be Blair's turn to appear.

The inquiry has identified five key issues: Blair's apparent determination to join George Bush in an invasion of Iraq to secure regime change with or without UN backing; the legality of the invasion; the Iraqi weapons dossier; military commanders being told to delay preparations for the invasion; and the failure to plan for the aftermath.

The common thread was the determination of Blair and his circle of close advisers to hide their true intentions, the evidence so far suggests.

Regime changeBlair's meeting with Bush at the president's ranch at Crawford, Texas, was a defining moment, the inquiry has heard. "I look back at Crawford as the moment that he [Blair] was saying, yes, there is a route through this that is an international, peaceful one and it is through the UN, but if it doesn't work, we will be willing to undertake regime change," Sir David Manning, Blair's chief foreign policy adviser, told the inquiry.

Campbell revealed that Blair privately assured Bush: "We are absolutely with you in making sure that Saddam Hussein is faced up to his obligations and that Iraq is disarmed. If that can't be done diplomatically and it has to be done militarily, Britain will be there."

Turnbull went further. He suggested that Blair started as a "regime changer" but then realised he had first to go down the UN route. At Crawford the two leaders decided to "set a trap" for Saddam in the form of a UN-backed ultimatum, Turnbull said.

Legality

Wilmshurst told the Foreign Office when she resigned that she could not support military action without a second UN resolution. That was also the view, she said, which Goldsmith held before changing his advice on the eve of the crucial Commons vote on the war.

Leaked documents show that Goldsmith warned Blair in July 2002 that regime change was "not a legal basis for military action".

The inquiry has heard that Goldsmith saw Blair privately about his legal opinion, which he drew up on 7 March 2003, warning that Britain could be indicted under international law if it invaded Iraq without a second UN resolution. The cabinet was not shown this. Instead, Goldsmith published a short statement on 17 March, the eve of the Commons vote, saying that military action without a second UN resolution was legal after all. This did not constitute a proper legal opinion, lawyers say.

The dossier

Campbell defended every word of the dossier, even Blair's assertion in the foreword that the assessed intelligence had "established beyond doubt" that Saddam was continuing to produce chemical and biological weapons. Lord Butler, one of Turnbull's predecessors as cabinet secretary, has described Blair's claims as "disingenuous".

Delay in military preparation

Lord Boyce, head of the armed forces at the time of the Iraq invasion, told the inquiry he had been unable to prepare British troops properly for war because the government did not want the plans to become public knowledge. He said Hoon banned him from talking to senior officers responsible for getting supplies ready for war.

The inquiry has heard that as a result, orders for military equipment needed for Iraq were also delayed.

Failure to prepare for aftermath

All witnesses have expressed astonishment at the failure to plan for the aftermath of an invasion and the lack of intelligence about the state of Iraq.

There was "a touching belief [in Washington] that we shouldn't worry so much about the aftermath because it was all going to be sweetness and light", Edward Chaplin, head of the Middle East department of the Foreign Office at the time, told the inquiry.

Describing Iraq after the invasion, Lieutenant General Frederick Viggers, Britain's senior military representative in Baghdad in 2003, said: "It was rather like going to the theatre and seeing one sort of play and realising you were watching a tragedy as the curtains came back."

Witnesses have presented a picture of a supine cabinet which did not ask questions about the legality, the necessity, or the consequences of war.